In nuclear reactors, for example in pressurized water nuclear reactors, a fuel is used that can be constituted mainly of uranium oxide or of a mixture of uranium oxide and plutonium.
Nuclear fuel, which is enriched in fissile elements, for example in uranium-235 in the case of a fuel constituted by uranium oxide, is generally obtained by a process in which the final enrichment product is constituted by gaseous uranium hexafluoride UF6.
The UF6 is then converted into uranium oxide by oxidation using steam, for example.
Current processes producing the best results for converting uranium hexafluoride into uranium oxide are dry direct conversion processes which are conducted in apparatus comprising, in succession, a reactor provided with means for introducing UF6 and steam into a chamber of the reactor in which uranium oxyfluoride UO2F2 is formed from the UF6, and a rotary tube furnace in which the solid powdered uranium oxyfluoride UO2F2 is transformed into uranium oxide, the tube furnace being provided with heater means and means for introducing steam and hydrogen via the outlet portion of the rotary furnace as a counter-current to the powdered solid moving in the longitudinal direction of the furnace.
Uranium oxide powder principally constituted by the dioxide UO2 is recovered from the outlet of the rotary furnace, that powder then being conditioned in a conditioning unit before being used to produce sintered nuclear fuel pellets.
The process for transforming uranium oxyfluoride UO2F2 into uranium oxide is carried out by bringing a powdered solid into contact with a reactive gas mixture containing steam and hydrogen in particular.
Hydrofluoric acid HF is formed in the rotary furnace by oxidizing the sulfur oxyfluoride with steam.
The solid material circulating in the rotary furnace coming into contact with the reactive gas mixture is the seat of various chemical reactions that result in the formation of uranium oxide, and principally of the dioxide UO2; in particular, the chemical reactions indicated below occur:UO2F2+H2O→UO3+2HF3UO3→U3O8+½O2 U3O8+2H2→3UO2+2H2O
Thus, the composition of the continuously moving solid material in the furnace and the composition of the reactive gas mixture vary essentially in the longitudinal direction of the furnace along which the powdered solid material moves, with the gas mixture moving as a counter-current.
In order to control the chemical reactions in the furnace to the best possible extent, the heater elements of the furnace disposed at the periphery of the jacket of the rotary furnace are adjusted to obtain a regular temperature distribution in the longitudinal direction of the furnace.
However, that method of adjusting the temperature in the longitudinal axial direction of the furnace cannot effectively control the composition of the uranium oxides at the furnace outlet.
Adjusting the flow rates of the hydrogen and steam introduced via the outlet end portion of the rotary furnace cannot improve control of the conversion reactions in the furnace because the reactive gases are diluted in the furnace and because of the random nature of the distribution of the reactive gases obtained inside the furnace chamber.
Further, uranium dioxide UO2 can be produced from the oxide U3O8 via intermediate reactions during which different uranium oxides are obtained in accordance with the transformation sequence U3O8→U3O7→U4O9→UO2.
In general, no method is known for determining how the reactions between the reactive gas mixture and the oxyfluoride or uranium oxides moving along the rotary furnace are progressing in the longitudinal direction of movement of the substances inside the rotary furnace. Access to a graph of the progress of the reactions inside the rotary furnace would mean that the reactions could be manipulated to optimize the conversion process to obtain oxides with the desired composition at the furnace outlet.
In particular, in order to obtain green pellets with very high mechanical strength as measured by crush, microhardness or wear tests in which the uranium oxide powder is compressed, it has been observed that it is necessary to use oxide powders of a composition such that the atomic ratio of the oxygen over the uranium (O/U) is substantially higher than the ratios normally obtained with oxides from the outlet of a uranium oxyfluoride converting furnace, which oxides are constituted principally by uranium dioxide UO2.
To increase the O/U ratio, mixing certain proportions of particles of uranium dioxide UO2 obtained by dry conversion with particles of an oxide such as U3O8 has been proposed, for example. That method, which can increase the O/U ratio of the oxides used to produce fuel pellets, necessitates oxidizing the uranium oxide UO2 under perfectly controlled conditions in order to obtain the oxide U3O8, and then forming a homogeneous mixture of UO2 and U3O8 particles. Thus, that method of producing uranium oxide powders is complex.
Currently, no method is known that can control a reaction from an accurate determination of the progress of a chemical conversion reaction in a furnace to obtain uranium oxides at the furnace outlet with the desired composition, and in particular uranium oxides with a high O/U ratio, i.e., oxides with a mean formula of the type UO2+x, where x is relatively high (x in the range 0.03 to 0.7, preferably in the range 0.05 to 0.25).
More generally, when a chemical reaction is carried out between a substance in a condensed state, for example a solid powdered substance moving continuously in the longitudinal direction of a furnace, and a reactive gas mixture, no method is known for accurately determining the extent of the reaction at different points along the longitudinal direction of the furnace, and no method is known for controlling the reaction in the furnace from any such accurate determination.
An accurate determination of the extent of the reaction in the furnace must be carried out without opening the furnace and without risking the introduction of air into the furnace interior, since that would both completely falsify the measurements and analyses carried out, and would also modify the product being produced in the furnace. Thus, it is not possible to monitor the extent of the reactions by removing samples of the moving powdered substance at different points along the axis and inside the furnace chamber.